Electrosurgery is a well-known and widely used technique for performing cutting and coagulation surgical operations. In order to perform an electrosurgery operation, the patient is connected to a electrical energy generator which produces high-frequency energy, generally in the frequency range of 100 kilohertz to 1 megahertz. The high-frequency energy is supplied to the patient at the operating area by means of an "active" electrode which has a small contact area with the patient. The high-frequency electrosurgical source is capable of producing a significant amount of current at a relatively high voltages and the high current density caused by the small contact area of the active electrode causes a localized cutting or coagulating action. The current, after flowing through the operation point, is returned to the high-frequency generator via an indifferent electrode or return plate. The current return point typically has a large contact area with the patient so that the density of current flowing from the patient to the plate is low at all contact points. The low current density prevents electrical burns from occurring at the point where the indifferent electrode contacts the patient.
Most prior art electrosurgery apparatus suffers from a common disadvantage in that the patient can suffer severe electrical burns if the electrosurgical current leaves the patient's body via a route other than the indifferent electrode. Surgical burns can be caused by secondary grounds which establish an alternative current path. If the area of the contact point at which the current leaves the patient's body is small, a burn can result. Secondary ground paths can occur over monitoring electrodes connected between the patient and grounded electrical monitoring equipment; additional ground paths can occur between the patient and a grounded support or operating table or between the patient and the surgeon.
Unfortunately, such burns can be quite severe because the patient is often unconscious during the surgical operation and therefore does not react. Consequently, burning can occur over a considerable period of time during which surgery is taking place.
In order to attempt to eliminate the problem of burns caused by alternate grounding paths, an electrosurgical generator that has an isolating output transformer is used. In this type of generator, the electrical power generated by the output stage of the generator is coupled to the active and indifferent electrodes by means of the secondary winding of a transformer which is not connected to the primary winding and is not grounded. Unfortunately, because of stray or leakage capacitance between the transformer windings and between the secondary winding and ground, the electrical isolation is far from perfect and severe patient burns can result if the return cable connecting the indifferent electrode plate to the electrosurgical source is broken or the patient moves out of contact with the indifferent electrode plate.
It is, therefore, an object of this invention to provide a cancelling circuit which cancels out leakage produced by improper grounding of the electrosurgical unit and prevents electrical patient burns.
It is a further object of this invention to provide a leakage cancelling circuit suitable for use with electrosurgery apparatus to prevent electrical burns.
It is another object of this invention to provide a leakage cancelling circuit capable of reducing the current flow through the patient at secondary ground points when an indifferent electrode connection to the patient is broken either because of inadequate patient contact to the indifferent electrode or because of a break in the line connecting the indifferent electrode to the electrosurgical generator.